Chromium-tungsten-tantalum alloys



United States Patent 3,138,457 CHROMIUM-TUNGSTEN-TANTALUM ALLOYS Alan R. Edwards, Surrey Hills, Victoria, Australia, assignor to Commonwealth of Australia,-Canberra, Australian Capital Territory, Commonwealth of Australia No Drawing. Filed Feb. 11, 1963, Ser. No. 257,797 2 Claims. 7 (Cl. *7517 6) This invention relates to chromium-tungsten-tantalum alloys and has for its object the provision of new alloys having good structural properties at high temperatures and particularly suitable for such applications as high performance gas turbine rotor blading.

Alloys according to the invention are characterised in that they consist essentially of l to 8% tungsten, 0.5 to

4% tantalum and 0.1 to 4% of a further alloying com- 3,138,457v Patented June 23, 1 96 4 taining 1 to 8% tungsten, 0.5 to 4% tantalum and 0.1 to 4% of one ormore of the additional elements titanium, niobium and silicon, with the balance notless than 85% chromium. There may also be impurities in the alloys, totalling not more than 0.5%,this total including gaseous impurities such as nitrogen and oxygen and non-gaseous impurities such as iron, nickel and aluminium. The amount of tungsten in the alloys is preferably in the range 1 to 6% and the amounts of titanium and niobium respectively are preferably in the range 0.1 to 2%.

At temperatures of 950 to 1100 C. alloys within the range specified have shown minimum creep rates and rupture lives under stress which compare favourably with these properties under similar or less severe conditions of a known high strength heat resistant alloy at present in commercial use as shown in the table which now follows:

Table Stress Test Minimum Rupture Alloy Condition (tons/ Temp creep rate Life Sq. in.) C.) (infirm/hr.) (hours) Commercial high strength heat resistantalloy, nickel, Solution treated 1,150 C. 6 050 120150 10- 23 chrom1um, 16% cobalt, 3% titanium, 2% aluand aged 700 C. 12 950 1,600X10- less than mimum. 5 Chromium, 2% tungsten, 2% tantalum, 0.5% silicon. Extruded 9 980 64 10- 586 Chromium, 2% tungsten, 2% tantalum, 0.5% silicon, do 9 980 21 10- 981 0.1% titanium. 12 980 223x10- 171 Chromium, 3% tungsten, 1% tantalum, 0.5% s1l1o0n 9 930 -a 519 Chromium, 3% tungsten, 1.5% tantalum, 0.5% silicon-.. 9 980 55 10- 328 Chromium, 4% tungsten, 1% tantalum, 0.5% Sll1C0l1.. 9 980 9 10- 160 Chr0 n1inm, 4% tungsten, 1% tantalum, 0.5% silicon, g 1 3? 01 mamum' 12 980 175 10- 242 ponent selected from the group consisting of titanium, niobium, silicon and mixtures thereof, and the balance chromium and incidental impurities, the chromium content amounting to not less than 85% by weight of the total alloy composition, and the impurities including gaseous impurities, iron, nickel, aluminium and other metals not exceeding a total of 0.5%.

The lower limit of 85 of the chromium content has 40 been found necessary to ensure that the alloys meet the following requirements for gas turbine rotor blading:

(a) An acceptably low density.-The tensile stress on rotor blading is directly proportional to the density of the alloy used, so that the accepted density for currently used turbine alloys is substantially less than 9 grammes/ cc. To meet this requirement, alloys of chromium with heavy metals must contain not less than 85% chromium.

(b) Good resistance to high temperature oxidation.

The naturally high oxidation-resistance of chromium is seriously impaired if it is alloyed with a total of more than 15% of poorly oxidation-resistant heavy metals.

(0) Acceptable hardness and machineability.-Alloys of chromium with heavy metals containing less than 85% chromium come into the category of hard metals, noted for their resistance to wear and to machining operations. It has, however, been ascertained that the alloys of the present invention, containing not less than 85% chromium, can be readily machined with normal machine tools.

Since the alloys are required to be finished to the close heavy metals can be formed by the accepted metalworking operations unless it contains at least chromium. However, the ability to be worked by extrusion, forging, rolling and like procedures is frequently required in alloys for turbine rotor blading and this ability is in general possessed by the alloys of the present invention. 70

It has been ascertained that satisfactory properties for the purpose of this invention are possessed by alloys con- The commercial alloy tested had a rupturelife. of approximately hours under a stress of 12 tons/ sq. in. at a temperature of 820 C., so that the alloys of the invention provide an advance of more than C. on the existing alloy.

In the titanium-bearing alloys of this invention, the titanium content may be wholly or partly substituted by niobium. Thus, for example, the following alloys show the same general level of high-temperature strength as those given in the table: 1

(l) Chromium, 2% tungsten, 2% tantalum, 0.5% silicon,

0.2% niobium.

(2) Chromium, 4% tungsten, 1% tantalum, 0.5% silicon,

0.05% titanium, 0.1% niobium.

While the primary strengthening addition in the alloys is the combination of tungsten and tantalum, the high temperature creep resistance is much enhanced by the further additions of titanium and niobium, and these also greatly improve the ductility and general handling characteristics of the alloys.

The main function of the silicon addition is to promote good resistance to high-temperature oxidation, but it may be omitted for applications in which this property is of less importance than good workability. Thus, for example, the following alloys show the same general level of high-temperature strength as those given in the table:

(1) Chromium, 2% tungsten, 2% tantalum, 0.1% titanium.

(2) Chromium, 4% tungsten, 1% tantalum, 0.2% niobium.

Alloys according to the invention can be hot-worked at high temperatures while a proportion of the alloys can be cold worked at lower temperatures.

For hot-working, high speed extrusion is employed, preferably using uncooled steel dies and a molten glass lubricant. The alloy is heated for extrusion in a protective atmosphere of hydrogen or an inert gas, such as argon or helium, to minimise surface contamination by atmospheric gases, particularly nitrogen. This heating method is also used for forging, which is conducted in the same temperature range. It is possible to produce a considerable range of shapes by hot-working. In particular, the extrusion method can be used to make approximate aerofoil sections, suitable for the production of turbine blades.

Thus it has been found possible to extrude all of the alloys shown in the table and other alloys in the specified range at temperatures above 1200 C. starting with a 1 /2" to 2" diameter arc-cast ingot and finishing with a A" to diameter rod. Small arc-cast ingots of alloys within the specified range and containing up to 10% tungsten can be press-forged at temperatures above 1200 C. to a useful extent using particular handling techniques and one forging operation per heat. The main feature of the handling techniques is the use of heated press platens which minimise the heat loss which would otherwise occur on transferring the ingots from the furnace to the press face.

The strength at high temperature of alloys according to the invention is such that cold-working must usually be conducted at temperatures well above room-temperature, in the range 500-1300 C.

Heat treatment, if employed, consists of an ageing treatment from 5 to 50 hours at the intended service temperature, or up to 200 C. above this temperature. The normal range for this treatment is 900 to 1200" C. This may or may not be preceded by a high-temperature solution treatment in the range 1200 to 1600" C.

The alloys according to the invention may be used in the cast condition, or in the hot-worked or cold-worked conditions, with or without heat-treatment. The materials are intended primarily for the rotor and stator blading of gas turbine engines, and in other applications requiring high load-carrying ability at temperatures of 900 C. and higher.

The alloys can be shaped by normal machining methods, using high-speed steel or carbide tools, or grinding wheels with a copious supply of cutting lubricant.

I claim:

1. A high strength heat resistant alloy consisting essentially of 1 to 8% tungsten, 0.5 to 4% tantalum and 0.1 to 4% of a further alloying component selected from the group consisting of titanium, niobium, silicon and mixtures thereof, and the balance chromium and incidental impurities, the chromium content amounting to not less than by weight of the total alloy composition, and the impurities including gaseous impurities, iron, nickel and aluminium not exceeding a total of 0.5%.

2. A high strength heat resistant alloy consisting essentially of 1 to 6% tungsten, 0.5 to 3% tantalum and 0.1 to 2% of a further alloying component selected from the group consisting of titanium, niobium, silicon and mixtures thereof, incidental impurities including gaseous impurities, iron, nickel and aluminium not exceeding a total of 0.5%, and the balance chromium.

References Cited in the file of this patent UNITED STATES PATENTS 2,491,866 Kurtz et al Dec. 20, 1949 2,977,225 Wlodek Mar. 28, 1961 3,067,031 Edwards Dec. 4, 1962 OTHER REFERENCES Periodic Table, Hackhs Chemical Dictionary, 3rd ed., The Blakiston Company, Philadelphia, Pa., 1944, page 632.

Rare Metals Handbook, C. A. Hampel, Reinhold Publishing Corp., 1954, pages 394 and 490. 

1. A HIGH STRENGTH HEAT RESISTANT ALLOY CONSISTING ESSENTIALLY OF 1 TO 8% TUNGSTEN, 0.5 TO 4% TANTALUM AND 0.1 TO 4% OF A FURTHER ALLOYING COMPONENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, NIOBIUM, SILICON AND MIXTURES THEREOF, AND THE BALANCE CHROMIUM AND INCIDENTAL IMPURITIES, THE CHROMIUM CONTENT AMOUNTING TO NOT LESS THAN 85% BY WEIGHT OF THE TOTAL ALLOY COMPOSITION, AND THE IMPURITIES INCLUDING GASEOUS IMPORITIES, IRON, NICKEL AND ALUMINIUM NOT EXCEEDING A TOTAL OF 0.5%. 